High strength steel sheet and method for producing the same

ABSTRACT

The present invention relates to A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3 μm or less. The secondary phase is constituted of martensite, or martensite and retained austenite.

TECHNICAL FIELD

The present invention relates to a high strength dual phase steel sheet having excellent elongation and stretch flangeability. In particular, it relates to a high strength dual phase steel sheet having excellent strength-elongation and stretch flangeability in high strength region of 590 MPa or more.

BACKGROUND OF THE INVENTION

In industrial fields of automobiles, electrical machinery and appliances, machinery and the like, steel sheets used by press forming are required to combine excellent strength and formability. In recent years, such demand characteristics are increasing more and more. Dual phase steel sheets (DP steel) having dual phase of ferrite and martensite, and TRIP steels comprising a structure containing ferrite, bainite and retained austenite are known as high strength steel sheets having excellent ductility. However, dual phase steel sheets such as DP steel and TRIP steel have the problem that stretch flangeability is poor.

To improve stretch flangeability in high strength steel sheets, a method of suppressing localization of strain by making the structure a single phase structure to homogenize processability in the structure, and a method of reducing strength difference between a soft phase and a hard phase in a dual phase structure are known. Regarding those two methods, for example, the following technologies are disclosed.

1. Single Phase Structure (See Patent Document 1)

A method of producing a martensite single phase steel sheet having tensile strength of from 880 to 1,170 MPa by bringing more balance to components and heat treatment conditions has been disclosed. For the production of a martensite single phase structure, soaking temperature necessary for austenitizing is set to 850° C. which is generally the industrially attainable temperature condition, and this made it possible to industrially achieve a martensite single phase structure. However, a steel sheet of a martensite single phase structure is that the structure is macroscopically uniform, so that stretch flangeability is excellent, but ductility (elongation) greatly deteriorates (EL<8%).

2. Reduction in Strength Difference of Dual Phase Structure (See Patent Document 2)

A steel sheet having a space factor of a low temperature transformation phase of 90% or more is heated and held in two phase regions of ferrite and austenite, thereby fine ferrite and austenite succeeding lath of a low temperature transformation phase can be formed. After the subsequent cooling, a structure in which ferrite and the low temperature transformation phase are finely dispersed in a lath state is finally formed. Such a finely dispersed low temperature transformation phase suppresses formation and growth of voids at the time of flange deformation, thereby improving stretch flangeability.

However, in this method, the structure become fine, but martensite which is a hard phase cannot be precipitated in grains. Furthermore, a grain size of martensite is 5 μm at the minimum, and the structure cannot sufficiently be homogenized. For this reason, improvement effect of stretch flangeability is not sufficient.

3. Fine Retained Austenite Steel Sheet (See Patent Document 3)

A steel sheet in which a secondary phase having an average grain size of 500 nm or less is finely dispersed in grains has been disclosed. High elongation and stretch flangeability are achieved by forming a starting point of fracture in the grains.

In this method, very expensive austenite stabilizing element such as Au, Ag or Ni must be added in order to form the secondary phase in the grains. Additionally, a concentrated region of the austenite stabilizing element must be formed in the grains in order to form the secondary phase in the grains, but to achieve this, a solution heat treatment at 1,270° C. or higher for 5 hours or more becomes necessary. Consequently, industrial problem in increase of time and cost remains in this method.

Patent Document 1: Japanese Patent No. 3729108

Patent Document 2: JP-A-2005-272954

Patent Document 3: JP-A-2005-179703

SUMMARY OF THE INVENTION

Accordingly, the present invention has an object to exhibit both elongation and stretch flangeability particularly in a high strength steel sheet of 590 MPa or more, and further to industrially realize such a high strength steel sheet.

To obtain a high strength steel sheet, it is necessary that a hard phase is a secondary phase, but when the hard phase is present at the grain boundary of a matrix, its interface becomes a starting point of fracture, and local elongation lowers. However, when a hard phase is not present, high strength cannot be obtained. When a hard phase can finely be present in the grains of a matrix, a starting point of fracture can be minimized, and as a result, local elongation of steel can be improved. It is known that local elongation of steel has a correlation with stretch flangeability thereof, and high stretch flangeability can be obtained by improving local elongation.

Thus, to improve stretch flangeability while maintaining high ductility in a high strength steel sheet of 590 MPa or more, it is important to be a dual phase steel sheet and to uniformly and finely precipitate a hard secondary phase in the grains of a matrix. Furthermore, it is important that the production of steel can industrially be realized.

In view of the above, the present inventors thought that the entire surface of a steel sheet is constituted of bainite in order to provide a steel sheet having improved stretch flangeability while maintaining high ductility even in a high strength region of 590 MPa or more, and thought to control the existence form of cementite in bainite. The reason that the matrix is bainite is that lath of bainitic ferrite constituting bainite is not grain boundary, and is therefore difficult to become a starting point of fracture. As a result of verification, cementite which is a hard phase could finely be dispersed among laths, but bainite is slightly poor in ductility, and the desired elongation was not obtained.

Subsequently, Si was added to improve elongation. By adding Si, precipitation of cementite is suppressed in the course of cooling, and austenite in which C is concentrated is formed among laths of bainitic ferrite. Furthermore, bainite transformation is completed, and cooling rate was controlled such that lath of bainitic ferrite disappears in the course of the subsequent cooling.

As a result, a structure comprising a ferrite phase and austenite finely dispersed therein was obtained in the cooling process. When austenite reached Ms point or lower, MD structure comprising a ferrite phase and martensite as a hard phase finely dispersed therein was obtained. However, in this method, a ferrite phase in which martensite is not dispersed in the ferrite phase was simultaneously formed. When such ferrite is present in the structure, strength deteriorates, and in addition, stretch flangeability deteriorats due to strength difference at the interface between the ferrite and MD.

In view of the above, the present inventors have further made researches and found that formation of ferrite which does not contain martensite can be suppressed by adding B. As a result, nearly the entire surface can be constituted of MD structure, and the present invention could be completed based on this finding.

When such unique components and heat treatment are employed, martensite having strength difference to a matrix smaller than a hard cementite can be uniformly and finely dispersed in ferrite grains. As a result, it was clarified that a dual phase steel sheet having excellent strength-stretch flangeability while maintaining high ductility can be provided even in a high strength region of 590 MPa or more.

Furthermore, DP steel, TRIP steel and the like described above are conventionally developed as the means to combine strength and ductility (elongation). DP steel comprises hard martensite and soft ferrite. The hard martensite ensures strength, and the soft ferrite ensures ductility (elongation). On the other hand, TRIP steel is that retained austenite generates deformation-induced transformation during deformation, thereby improving ductility (elongation). However, the ductility (elongation) used herein means uniform elongation, and those steel sheets are excellent to combine strength and ductility (uniform elongation), but had the disadvantage that local elongation is poor. It is known that local elongation has a high correlation with stretch flangeability, and improvement in local elongation is indispensable to obtain high stretch flangeability.

Accordingly, as a result of intensive investigations to provide a steel sheet which combines elongation and stretch flangeability even in a high strength region of 590 MPa or more, namely a steel sheet combining uniform elongation and local elongation in addition to strength, the present inventors have succeeded to develop a high strength steel sheet having excellent elongation and stretch flangeability, leading to the present invention.

Development process of the steel of the present invention and mechanism of exhibiting high properties are described below.

To combine strength and ductility, it is advantageous to be a dual phase structure in which a soft phase and a hard phase are combined, and the steel of the present invention has a dual phase structure comprising ferrite as a soft phase and martensite as a hard phase. However, even though a dual phase structure merely comprising soft ferrite and hard martensite is formed as in DP steel as described before, it is effective to combine strength and uniform elongation, but it is not sufficient to ensure excellent stretch flangeability. The reason that DP steel is poor in stretch flangeability includes size and existence position of martensite as a hard phase. Specifically, martensite in DP steel is generally large as about several ten μm, and grain boundary between martensite and ferrite is large tilt grain boundary. When deformation is added to a steel sheet, stress concentrates at grain boundary (large tilt grain boundary) as compared with the inside of grains. When interface having strength difference is present here, voids are generated in a moment, which is liable to lead to fracture. Such generation of voids and fracture have a correlation with local elongation, and a steel sheet that is liable to cause generation of voids and propagation of cracks is poor in local elongation. Grain boundary between ferrite and martensite in DP steel is just an interface having strength difference, and excellent stretch flangeability cannot be possessed for this reason. Furthermore, although it is considered that this tendency is decreased with decreasing a size of a hard phase, this effect cannot be obtained in a size of martensite in general DP steel.

In view of the above, the present inventors thought that as the means for improving stretch flangeability while maintaining excellent properties of DP steel (combining strength and uniform elongation), a dual phase structure having a soft phase (ferrite) and a hard phase (martensite) is basically formed, but martensite is finely dispersed in ferrite grains (or small tilt grain boundary). They further thought that even in the case of ensuring the same degree of strength, grain size of the individual martensite can be decreased as compared with the general DP steel, and by further making martensite present in grains (or small tilt grain boundary), it becomes possible to suppress generation of voids in the case that deformation has been generated. By those, it is possible to combine elongation and stretch flangeability, and in addition to this, it is expected that ductility (particularly, uniform elongation) is further improved by utilizing TRIP effect as a structure containing retained austenite. The steel of the present invention developed based on the above-described thoughts has a structure comprising ferrite as a matrix, and martensite and retained austenite, finely dispersed mainly in ferrite grains. Due to this structure, the steel combines elongation and stretch flangeability in addition to strength.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view showing thermo-mechanical treatment in the Examples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the following (1).

(1) A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix,

wherein the proportion that the MD structure occupies in the whole structure is 90% or more,

wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%,

wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and

wherein the average grain size of the secondary phase in the whole structure is 3 μm or less.

In the present invention, the secondary phase is constituted of martensite, or martensite and retained austenite.

The term “proportion” used herein means a proportion of area (area fraction). Furthermore, the term “whole structure” used herein means the entire of MD structure and other structures.

It is desirable that an average grain size of ferrite in the whole structure is 20 μm or less, and 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.

Furthermore, it is desirable in the high strength steel sheet that the proportion that the retained austenite present in the whole structure occupies in the whole structure is 2% or more.

The high strength steel sheet has a composition comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.

The high strength steel sheet may further contain one or two or more of:

(1) Mo: 0.03 to 1%;

(2) at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%;

(3) at least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%);

(4) Cr: 1.5% or less (not including 0%); and

(5) at least one of Ca: 0.003% or less (not including 0%) and REM (rare earth element): 0.003% or less (not including 0%).

In addition, a method for producing a high strength steel sheet according to the present invention comprises heating a steel sheet material comprising the above-described component compositions, and cooling the steel sheet material from a temperature of A3 point or higher to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec.

The steel sheet material is produced by appropriately conducting a hot rolling step and a cold rolling step. Those steps are not particularly limited, and the conditions generally practiced can appropriately be selected and employed. For example, as the hot rolling step, conditions of holding at about 1,200° C. for 30 minutes, conducting hot rolling at A3 point or higher, cooling in an average cooling rate of about 30° C./sec, and winding up at about 500 to 600° C. can be employed. In the cold rolling step, it is recommended to conduct cold rolling in a cold rolling ratio of about 30 to 70%, but it is not particularly limited to this.

After heating the steel sheet material at a temperature of A3 point or higher and holding the same at that temperature, the steel sheet material is cooled to a temperature of Ms point or lower, generally room temperature. As a result, the structure of the present invention comprising MD structure in major portion can be obtained, and stretch flangeability can be improved while maintaining high ductility, even in a high strength region of 590 MPa or more.

When the steel sheet material is heated to a temperature of A3 point or higher and held at that temperature, the entire surface of the structure converts into austenite. Then, when the steel sheet material is cooled in a cooling rate of 0.2 to 20° C./sec, a dual phase structure of ferrite-pearlite is generally obtained. However, because Si is added in the present invention, precipitation of cementite is suppressed, and as a result, bainitic ferrite begins to precipitate from the austenite interface. With the growth of lath of bainitic ferrite, austenite decreases its space factor, and austenite is finely dispersed among laths of bainitic ferrite.

The lath of bainitic ferrite is thermally unstable, and therefore disappears in the cooling process after completion of the bainite transformation. As a result, the state that austenite is finely dispersed in the grains of ferrite having bainite block size is obtained, and when the temperature becomes Ms point or lower by further cooling, austenite transforms into martensite. As a result, MD structure comprising ferrite matrix formed by disappearance of the lath of bainitic ferrite, and a secondary phase comprising martensite finely dispersed in the matrix can be obtained. When B is further added, formation of ferrite which does not contain martensite in the grains can be suppressed. Furthermore, when Mo is added, bainite transformation can be completed in a short period of time. Mo is effective to allow the lath of bainitic ferrite to disappear.

Furthermore, the steel sheet having the above-described structure morphology can be obtained by optimizing chemical compositions and production steps (thermo-mechanical treatment conditions) in an industrially possible range. Unclear points still remain regarding appearance mechanism of the above-described structure, and all is not always clarified to the detail. However, at the present, it is considered that the following explanation is possible.

The steel of the present invention can be produced by heating a steel sheet material comprising the above-described component composition controlled in an appropriate range, cooling the material from a temperature of A3 point or higher, subjecting the material to a processing of rolling reduction of 5% or more in a temperature range of 600 to 1,000° C., and then cooling the material to a temperature of Ms point or lower in a cooling rate of 0.2 to 20° C./sec. This production step (thermo-mechanical treatment) may be conducted in a hot rolling step or may be conducted after a hot rolling step and a cold rolling step, generally conducted (in this case, this thermo-mechanical treatment is conducted by reheating).

Accordingly, MD structure comprising ferrite as a matrix, and martensite and retained austenite, which are finely dispersed mainly in ferrites grains is formed. Thus, the structure of the present invention can be produced by forming an austenite single phase structure at a temperature of A3 point or higher, cooling, adding processing at a predetermined temperature and then cooling in an appropriate range.

As described above, the structure formation mechanism of the steel of the present invention is not clarified to the detail, but one explanation that is considered at the present is described below. Therefore, the following description is not construed to limit the technical scope of the present invention.

One of various structure design guidelines devised to obtain the structure of the present invention is described below.

The present inventors thought to obtain a structure comprising ferrite as a matrix, and martensite and retained austenite finely dispersed therein, using bainite transformation generated during the cooling process. For this, bainite is formed during the cooling step after processing, but in the steel of the present invention, a mixed structure of bainitic ferrite and cementite as in general bainite is not formed. Since cementite is harder than martensite, such is harmful to stretch flangeability. Furthermore, to ensure retained austenite which is essential in the steel of the present invention, it is necessary to concentrate C in austenite, and it is therefore necessary to prevent precipitation of cementite which disturbs to concentrate C. For this reason, in the steel of the present invention, Si and Al are added to suppress precipitation of cementite in the cooling process. Accordingly, austenite in which C is concentrated among laths of bainitic ferrite is formed. This austenite is transformed into martensite in the subsequent cooling step, or remains as retained austenite without transformation.

Lath of bainitic ferrite is small tilt grain boundary, and concentration of strain during deformation as in large tilt grain boundary described before is difficult to be generated. As a result, martensite and retained austenite present among laths of bainitic ferrite are difficult to become a starting point of fracture. The term “ferrite grain boundary” used herein means large tilt grain boundary of ferrite, and lath boundary which is difficult to become a starting point of fracture is not grain boundary.

However, it is difficult to observe small tilt grain boundary at the position that martensite and retained austenite are present, in the structure obtained by the present inventors. It suggests that the steel of the present invention has further excellent structure morphology based on the following reasons.

In the steel of the present invention, processing is added at a predetermined temperature region, and as a result, bainite transformation is accelerated and bainite transformation is completed in a short period of time. Additionally, since the subsequent cooling rate is set to relatively low condition, it is still present at relatively high temperature for a certain period of time even after bainite transformation. For this reason, it is considered that even lath (small tilt grain boundary) of bainitic ferrite disappears, and as a result, it is considered that a structure (MD structure) comprising ferrite grains and martensite and retained austenite mainly dispersed therein was obtained.

In the above description, one of thoughts considered at present is shown as appearance mechanism of the structure of the present invention, but only the steel comprising the bainitic ferrite as a matrix is not the object of the present invention. In other words, ferrite which is a matrix of MD structure of the present invention is not limited to only ferrite wherein lath of bainitic ferrite becomes unclear, but includes ferrite which is precipitated at relatively high temperature, like pro-eutectoid ferrite. Thus, the present invention includes all of MD structure steel sheets having structure morphology defined in the present application.

Furthermore, the present inventors have found in the course of the above investigations that when ferrite in which martensite and retained austenite are not present is present, not only strength deteriorates, but stretch flangeability deteriorates by strength difference at the interface between ferrite and MD structure. Based on this finding, formation of such martensite-free ferrite is suppressed by adding B, thereby nearly the entire surface is constituted of MD structure.

The present invention can provide a dual phase steel sheet having excellent strength-elongation and stretch flangeability while maintaining high ductility even in high strength region of 590 MPa or more by forming a structure comprising martensite, or martensite and retained austenite which have small strength difference to a matrix as compared with hard cementite, uniformly and finely dispersed in ferrite grains.

Furthermore, according to the present invention, such a dual phase steel sheet can be produced by industrially possible means.

The structure of the present invention in the case that the secondary phase is martensite is described below.

Ferrite which is a matrix means ferrite having martensite contained in grains, formed in the cooling process from A3 point, and this ferrite includes pro-eutectoid ferrite and ferrite from which lath of bainitic ferrite has disappeared. On the other hand, this ferrite does not include ferrite which does not contain fine martensite in grains even though it is pro-eutectoid ferrite and ferrite from which lath of bainitic ferrite has disappeared. In the present invention, ferrite containing martensite in grains and ferrite which does not contain fine martensite in grains can be identified by, for example, applying the structure after completion of cooling to repeller corrosion, and showing ferrite in gray and martensite in white by image analysis. The entire surface of ferrite which does not contain fine martensite in grains is gray. On the other hand, the MD structure of the present invention is that fine martensite is contained in a ferrite phase. Therefore, fine white spot can be confirmed in ferrite grains.

The structure of the present invention is mainly composed of MD structure, and the MD structure comprises ferrite as a matrix and martensite as a secondary phase. The martensite as a secondary phase is formed such that austenite present in the ferrite matrix transforms into martensite at Ms point or lower in the cooling process. When the area fraction of the whole structure is defined as 100%, the proportion of the MD structure is required to be 90% or more. Where the proportion of the MD structure is less than 90%, influence of the remainder increases, uniformity of the structure is impaired, and stretch flangeability deteriorates. The remainder includes retained austenite, pearlite, bainite, and ferrite which is not accompanied with martensite in grains. On the other hand, the proportion that the MD structure occupies in the whole structure is better as large, and the upper limit is not particularly defined. Furthermore, when the area fraction of the whole structure is defined as 100%, the proportion of martensite as a secondary phase present in the whole structure (martensite present in MD structure and other structures) is required to be from 10 to 60%, and the desired strength can be obtained by this proportion. When the proportion is less than 10%, sufficient strength is not obtained, and when it exceeds 60%, elongation deteriorates. The proportion is preferably in a range of from 20 to 60%, more preferably in a range of from 30 to 55%, and most preferably in a range of from 40 to 50%.

Martensite in the MD structure is nearly uniformly dispersed in ferrite grains and at ferrite grain boundary. When the whole of the martensite is defined as 100%, the proportion of martensite in ferrite grains is required to be 50% or more (namely, the proportion of martensite present at ferrite grain boundary is less than 50%), and the desired stretch flangeability can be obtained by this proportion. When the proportion is less than 50%, martensite present at ferrite grain boundary acts as a starting point of fracture, and stretch flangeability deteriorates. The proportion of martensite in ferrite grains is preferably 60% or more, and more preferably 70% or more.

In the present invention, the term “grain boundary of ferrite matrix” means block boundary of bainitic ferrite formed by bainite transformation during cooling, and, for example, when orientation difference of adjacent ferrites is measured by FE/SEM-EBSP (Electron Back Scatter Diffraction Pattern), large tilt grain boundary in which the orientation difference is 15° or more is defined as block boundary, i.e., grain boundary, and a region surrounded by the block boundary is defined as grain.

Next, the structure of the present invention in the case that the secondary phase is martensite and retained austenite is described below.

The structure of the steel of the present invention is mainly composed of a structure (MD structure) comprising ferrite grains as a matrix and contained therein, fine martensite and retained austenite, as a secondary phase. Ferrite which does not contain a fine secondary phase in grains and MD structure differ in structure morphology. Furthermore, influence giving to properties differs between those, and it is therefore necessary to distinguish those.

As the identification method, for example, those can be identified by, for example, subjecting a steel sheet containing those structures to repeller corrosion, and showing ferrite in gray and martensite and retained austenite in white by image analysis. The entire surface of ferrite which does not contain the fine secondary phase in grains is gray. On the other hand, the MD structure is that fine martensite and retained austenite are contained in a ferrite phase. Therefore, fine white spot can be confirmed in ferrite grains.

When the area fraction of the whole structure is defined as 100%, the proportion of the MD structure (ferrite+martensite+retained austenite) is required to be 90% or more. When the proportion of the MD structure is less than 90%, the influence of the remainder increases, uniformity of the structure is impaired and stretch flangeability deteriorates. The remainder includes ferrite which is not accompanied with the secondary phase in grains, pearlite and bainite. On the other hand, the proportion that the MD structure occupies in the whole structure is better as large, and the upper limit thereof is not particularly defined.

When the area fraction of the whole structure is defined as 100%, the proportion of martensite and retained austenite as a secondary phase present in the whole structure (martensite and retained austenite present in MD structure and other structures) is required to be from 10 to 60%, and the desired strength can be obtained by this proportion. When the proportion is less than 10%, sufficient strength is not obtained, and when it exceeds 60%, elongation deteriorates. The proportion is preferably in a range of from 15 to 55%, more preferably in a range of from 20 to 50%, and most preferably in a range of from 20 to 35%.

The proportion of the secondary phase (martensite and retained austenite in MD structure) in ferrite matrix is that when the whole of the secondary phase in MD structure is defined as 100%, the proportion of the secondary phase in ferrite grains is required to be 50% or more (the proportion of martensite and retained austenite on grain boundary is less than 50%), and the desired stretch flangeability can be obtained by this proportion. When the proportion is less than 50%, martensite and retained austenite present on the grain boundary act as a starting point of fracture, and stretch flangeability deteriorates. The proportion is better as large, and is preferably 60% or more, and more preferably 70% or more.

In the present invention, the term “grain boundary” means large tilt grain boundary, and for example, when orientation difference of adjacent grains is measured by FE/SEM-EBSP (Electron Back Scatter Diffraction Pattern), large tilt grain boundary in which the orientation difference is 15° or more is defined as grain boundary.

When the area fraction of the whole structure is defined as 100%, the proportion that retained austenite present in the whole structure (retained austenite present in MD structure and other structures) occupies in the whole structure is desirably 2% or more. When retained austenite is present as the secondary phase, the desired ductility (mainly uniform elongation) can be obtained. When the proportion is less than 2%, the expected ductility improvement effect is diluted. The proportion is preferably 5% or more, and more preferably 8% or more. The proportion of the retained austenite is better as large, but from a practical standpoint, the upper limit is about 20%.

In the structure of the present invention, an average grain size of the secondary phase present in the whole structure (the secondary phase present in MD structure and other structures) is required to be 3 μm or less. When the average grain size of the secondary phase is larger than 3 μm, starting points of fracture are localized, and as a result, sufficient stretch flangeability is not obtained. The average grain size of the secondary phase is better as small, and is preferably 2.5 μm or less, and more preferably 2 μm or less.

An average grain size of ferrite in the whole structure (ferrite present in MD structure and other structures) is preferably 20 μm or less, and the desired stretch flangeability can be obtained by this grain size. When the grain size exceeds 20 μm, local deformability deteriorates, and stretch flangeability deteriorates, which is not preferable. The average grain size is better as small, and is preferably 15 μm or less, and more preferably 10 μm or less.

On the other hand, the secondary phase in the whole structure (martensite, or martensite and retained austenite, present in MD structure and other structures) is required to be finely dispersed, and it is preferred that 20 or more secondary phases on the average are observed in an observation view of 50 μm×50 μm. When the number of the secondary phase is less than 20, the desired strength may not be ensured, or there is the possibility that stretch flangeability deteriorates, which is not preferable. The number of the secondary phase is preferably 30 or more, and more preferably 40 or more. The number is preferable as large, such as 50 or more, 80 or more, and 100 or more.

The MD structure steel sheet having the structure morphology of the present invention has a composition comprising, in terms of % by mass, C: 0.02 to 0.3%, Si: 0.01% to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.

The composition may further contains one or two or more of:

(1) Mo: 0.03 to 1%;

(2) at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%;

(3) at least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%);

(4) Cr: 1.5% or less (not including 0%); and

(5) at least one of Ca: 0.003% or less (not including 0%) and REM (rare earth element): 0.003% or less (not including 0%).

The above component composition is described below. In the followings, all the percentages are defined by mass, unless otherwise indicated. Additionally, all the percentages defined by mass are the same as those defined by weight, respectively.

C: 0.02 to 0.3%

C is an effective component to obtain strength of steel. The lower limit 0.02% is the minimum necessary amount to obtain a predetermined space factor of a secondary phase, thereby obtaining the desired strength. The upper limit 0.3% is that where C is added more than this amount, strength of martensite increases, and as a result, stretch flangeability deteriorates. The content of C is in a range of preferably from 0.03 to 0.25%, and more preferably from 0.04 to 0.20%.

Si: 0.01 to 3%

Si is an element to decrease the amount of a solid solution C in ferrite, to contribute to improvement of ductility such as elongation, and to suppress precipitation of cementite during bainite transformation. Si is added in an amount of 0.01% or more. When the amount of Si added is less than 0.01%, cementite precipitates during cooling, and martensite is not formed among laths. Additionally, it is difficult to ensure the desired amount of retained austenite. The content of Si is preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, when Si is added in an amount exceeding 3%, cracks may be generated, resulting in deterioration of elongation and stretch flangeability. The content of Si is preferably 2.5% or less, and more preferably 2% or less.

Mn: 0.5 to 3%

Mn enables steel to have high strength by solid-solution hardening, and at the same time, improves hardenability of steel. Mn further has a function to accelerate formation of martensite. Such a function is recognized in the steel having Mn content of 0.5% or more. Mn content is preferably 0.7% or more, and more preferably 1% or more. On the other hand, when Mn is added in an amount exceeding 3%, stretch flangeability deteriorates. Mn content is preferably 2.5% or less, and more preferably 2% or less.

B: 0.0001 to 0.005%

B is important for the present invention. When B is added, formation of pro-eutectoid ferrite can be suppressed, thereby preventing precipitation of ferrite which is not accompanied with fine secondary phase in grains, such ferrite adversely affecting stretch flangeability. This effect is exhibited when B is added in an amount of 0.0001% or more. The amount of B added is preferably 0.0002% or more, and more preferably 0.0003% or more. On the other hand, when B is added in an amount exceeding 0.005%, the degree of segregation to grains increases, resulting in deterioration of stretch flangeability. For this reason, 0.005% is defined as the upper limit of B content. The amount of B is preferably 0.004% or less, and more preferably 0.003% or less.

Al: 0.01 to 1.5%

Al is used for deoxidation of steel. When the amount of Al is less than 0.01%, silicate inclusions retain, thereby resulting in deterioration of processability of steel. Therefore, the amount of Al added must be 0.01% or more. Furthermore, similar to Si, Al has the effect to prevent precipitation of cementite, and to contribute to ensuring retained austenite. The amount of Al added is preferably 0.03% or more, and more preferably 0.05% or more. However, the amount of Al exceeding 1.5% invites increase of surface mark, and retained austenite and martensite are liable to grow, inviting deterioration of elongation and stretch flangeability. For this reason, the upper limit of Al content is set at 1.5%. The amount of Al is preferably 1% or less, and more preferably 0.5% or less.

The steel sheet material of the present invention may further contain the following components in addition to the above-described essential components.

Mo: 0.03 to 1%

Mo is an element effective to accelerate bainite transformation and to accelerate disappearance of lath of bainitic ferrite during heat treatment. Mo further has the effect to suppress ferrite transformation and to improve hardenability. To obtain such effects, Mo is necessary in an amount of 0.03% or more. The amount of Mo is preferably 0.07% or more, and more preferably 0.1% or more. On the other hand, addition of Mo in an amount exceeding 1% results in increase of strength and deterioration of stretch flangeability. Therefore, the upper limit of Mo content is set at 1%. The amount of Mo is preferably 0.8% or less, and more preferably 0.6% or less.

Total of at least one element selected from the group consisting of Nb, Ti and V: 0.01 to 0.1%

Nb, Ti and V have the function to form a carbonitride and to increase strength of steel by precipitation strengthening and the function to refine grains, and can be added according to need. Such functions are not exhibited when the total amount of at least one of Nb, Ti and V added is less than 0.01%. On the other hand, when at least one of those is added in the total amount exceeding 0.1%, precipitate increases, resulting in remarkable deterioration of stretch flangeability. For this reason, the upper limit is set at 0.1%. The total amount is preferably 0.05% or less, and more preferably 0.03% or less.

At least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%)

Ni and Cu are elements effective to realize high strength while maintaining high strength-ductility balance, and are appropriately added. However, even though those are added in excess, the effect is saturated, and additionally, productivity deteriorates, such as generation of cracks during hot rolling. Therefore, the amounts of Ni and Cu added are suppressed to 0.5% or less, respectively. The amounts of those added are preferably 0.3% or less, and more preferably 0.2% or less, respectively. To effectively exhibit the effect, it is recommended to add Ni in an amount of 0.1% or more and/or Cu in an amount of 0.1% or more.

Cr: 1.5% or less (not including 0%)

Cr is an element effective to enhance strength of steel by improving hardenability, and is appropriately added. However, even though it added in excess, the effect is saturated, and additionally, ductility deteriorates. Therefore, the amount of Cr added is preferably suppressed to 1.5% or less. The amount of Cr added is preferably 1.0% or less, and more preferably 0.8% or less. To effectively exhibit the effect, it is recommended to add Cr in an amount of 0.1% or more.

At least one of Ca: 0.003% or less (not including 0%) and REM: 0.003% or less (not including 0%)

Ca and REM (rare earth element) are elements effective to control morphology of a sulfide in steel and to improve stretch flangeability, and are appropriately added. However, even though those are added in excess, the effect is saturated, and the excessive addition is not economical. Therefore, the amounts of Ca and REM added are 0.003% or less, respectively. To effectively exhibit the effect, it is recommended to add those in an amount of 0.0003% or more, respectively. Examples of REM include Sc, Y and lanthanoid.

Inevitable Impurities

The composition of the steel sheet according to the present invention is that the remainder other than the above-described components comprises Fe and inevitable impurities. Of the inevitable impurities, P and S are admitted to contain in an amount of P: 0.07% or less (not including 0%) and S: 0.07% or less (including 0%). When processability of a steel sheet is considered, the amount of P and S added is better as small. In particular, where the content of S is high, an inclusion (MnS) increases, which remarkably adversely affects stretch flangeability of a steel sheet. However, where S is added in the above range, the addition does not give influence to properties of a steel sheet.

Next, in the steel sheet according to the present invention, production conditions for obtaining the above-described structure are described below.

The steel sheet of the present invention can be produced by conducting a hot rolling step and a cold rolling step to produce a steel sheet material, and applying a heat treatment step to the steel sheet material. Specific processes of the hot rolling step and the cold rolling step are as described before. However, the production conditions are not limited to those steps, and may appropriately be according to the necessary processes.

The purpose of the heat treatment step is to finely and uniformly disperse martensite in a ferrite phase. When the heating temperature is lower than A3 point, the whole structure does not convert into austenite during heating and holding, and ferrite which is not accompanied with martensite in the state before the heat treatment remains partially. This ferrite has low strength, and forms an interface having large strength difference to MD. As a result, stretch flangeability deteriorates. When the heating temperature is A3 point or higher, such ferrite all disappears. Therefore, the upper limit of the heating temperature is not particularly defined, but from the relationship with practical operating level, it is recommended to appropriately control the heating temperature to a proper value.

To obtain a predetermined structure of the present invention by converting a major part of the structure into MD structure, in the above-described steel composition, it is necessary to set a cooling rate to a range of from 0.2 to 20° C./sec, and cool to Ms point or lower. In general, it is cooled to room temperature. When the cooling rate is less than 0.2° C./sec, ferrite which is not accompanied with martensite as a secondary phase is formed, and the proportion of ferrite and martensite in the MD structure becomes less than 90%, resulting in deterioration of strength and stretch flangeability. Furthermore, the proportion of martensite in ferrite grains decreases, and growth of an average grain size of martensite is generated. The cooling rate is preferably 0.5° C./sec or more, and more preferably 1° C./sec or more. On the other hand, when the cooling rate exceeds 20° C./sec, time and temperature sufficient to disappear lath of bainitic ferrite cannot be ensured, and as a result, elongation and stretch flangeability deteriorate. Where the cooling rate further increases, the whole structure is converted into martensite, and as a result, elongation remarkably deteriorates. For this reason, the cooling rate is required to be 20° C./sec or less. The cooling rate is preferably 15° C./sec or less, and more preferably 10° C./sec or less.

Next, in the steel sheet according to the present invention, other production conditions for obtaining the structure are described below.

The high strength steel sheet of the present invention, that is, MD steel sheet, can be obtained by heating a steel sheet material satisfying the component system as defined above, cooling the material from a temperature of A3 point or higher (for example, 1,200° C.), subjecting the material to a processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C., and then cooling the material to Ms point at a cooling rate of from 0.2 to 20° C./sec.

Heating and holding the steel sheet material at a temperature of A3 point or higher, and then subjecting the material to a processing at rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C.:

The steel sheet of the present invention has MD structure comprising ferrite as a matrix and a secondary phase finely dispersed therein, on nearly the entire surface thereof. When the heating temperature is lower than A3 point, the structure during heating does not converts into austenite on the entire surface, and ferrite which is not accompanied with fine secondary phase is formed. The ferrite which is not accompanied with the fine secondary phase in grains contributes to the increase of elongation, but strength difference at the interface to the MD structure increases, and as a result, stretch flangeability deteriorates. The heating temperature is sufficient to be A3 point or higher, but from the relationship with a practical operating level, it is recommended to appropriately control the heating temperature to a proper value.

The subsequent addition of the processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C. is, for example, to accelerate bainite transformation to thereby facilitate obtaining fine MD structure, or to decrease an average grain size of ferrite. When the processing temperature is lower than 600° C., ferrite is precipitated, and when it is higher than 1,000° C., an average grain size of ferrite increases. Furthermore, where processing ratio is lower than 5%, bainite transformation is not accelerated. The upper limit of the processing ratio is practically about 90%, but the processing ratio higher than that has no problem. The processing temperature is preferably in a range of from 620 to 980° C., and more preferably in a range of 650 to 950° C. The rolling reduction is preferably in a range of from 8 to 85%, more preferably from 10 to 80%, and further preferably from 10 to 60%.

Cooling the steel sheet material to room temperature at a cooling rate of from 0.2 to 20° C./sec:

To obtain a predetermined structure of the present invention by converting a major part of the structure into MD structure, in the above-described steel composition, it is necessary to set a cooling rate to a range of from 0.2 to 20° C./sec. In general, it is cooled to room temperature. When the cooling rate is less than 0.2° C./sec, ferrite which is not accompanied with a secondary phase in grains is formed, and the proportion of the MD structure becomes less than 90%, resulting in deterioration of strength and stretch flangeability. The cooling rate is preferably 0.5° C./sec or more, and more preferably 1° C./sec or more. On the other hand, where the cooling rate exceeds 20° C./sec, the whole structure converts into martensite, and as a result, elongation remarkably deteriorates. For this reason, the cooling rate is required to be 20° C./sec or less. The cooling rate is preferably 15° C./sec or less, and more preferably 10° C./sec or less.

EXAMPLE

Examples in the case that the secondary phase is martensite are described below.

Steels 1A to 5F having the component compositions shown in Tables 1 to 5 were melt to form slabs. The slabs were heated to 1,150° C., hot rolled to a sheet thickness of 3.0 mm at 800° C. and then taken up at 550° C. Surface scale was removed by acid pickling, followed by cold rolling to a sheet thickness of 1.2 mm. Each of the steel sheet materials thus obtained was heated and held at each temperature shown in Tables 6 to 9, and then subjected to heat treatment which cools to each stop temperature at a cooling rate shown in Tables 6 to 9.

TABLE 1 Chemical component of specimen Chemical component (mass %) Steel code C Si Mn P S Al B Mo Other Ac3 (° C.) Ms point (° C.) Remarks 1A 0.008 0.69 1.89 0.008 0.005 0.23 0.0009 0.87 950 476 Comparison 1B 0.03 1.71 1.22 0.01 0.005 0.17 0.0008 0.56 969 494 Invention 1C 0.03 1.55 2.35 0.008 0.004 0.03 0.0005 0 944 448 Invention 1D 0.08 1.28 1.31 0.009 0.006 0.13 0.0012 0.06 912 474 Invention 1E 0.07 0.62 0.67 0.01 0.005 0.26 0.0034 0 884 506 Invention 1F 0.011 0.09 0.84 0.01 0.005 0.61 0.0011 0.13 897 531 Invention 1G 0.09 1.07 1.17 0.007 0.004 0.95 0.0019 0 897 500 Invention 1H 0.13 2.43 1.67 0.008 0.005 0.05 0.0016 0.45 960 437 Invention 1I 0.12 1.61 2.27 0.01 0.005 0.08 0.0007 0 912 421 Invention 1J 0.18 0.58 1.34 0.008 0.005 0.13 0.0009 0.23 857 436 Invention 1K 0.19 2.17 0.66 0.01 0.006 0.35 0.0015 0 919 466 Invention 1M 0.09 0.008 1.78 0.009 0.005 1.09 0.0018 0.56 867 478 Comparison 1N 0.05 0.02 2.21 0.011 0.005 0.23 0.0023 0.87 893 448 Invention 1O 0.04 0.03 1.25 0.01 0.007 0.81 0.0036 0.13 875 510 Invention 1P 0.08 0.046 1.81 0.01 0.005 0.33 0.0012 0.08 857 460 Invention 1Q 0.13 0.052 1.19 0.009 0.006 0.76 0.0018 0 839 479 Invention 1R 0.06 1.32 1.42 0.011 0.005 0.09 0.0008 0.32 929 474 Invention 1S 0.08 1.34 1.46 0.01 0.005 0.12 0.0009 0 912 468 Invention 1T 0.07 1.23 1.51 0.009 0.006 0.22 0.0015 0.28 920 471 Invention 1U 0.18 1.81 2.34 0.01 0.005 0.07 0.0028 0 905 396 Invention 1V 0.07 2.35 1.02 0.011 0.005 0.34 0.0019 0.34 972 493 Invention 1W 0.12 2.33 1.91 0.007 0.004 0.67 0.0007 0 944 452 Invention 1X 0.04 2.68 1.54 0.01 0.005 0.58 0.0035 0.32 999 490 Invention 1Y 0.17 2.76 1.83 0.009 0.005 0.32 0.0018 0 950 427 Invention 1Z 0.08 3.34 0.76 0.011 0.006 0.87 0.0009 0.04 995 517 Comparison

TABLE 2 Chemical component of specimen Chemical component (mass %) Steel code C Si Mn P S Al B Mo Other Ac3 (° C.) Ms point (° C.) Remarks 2A 0.16 1.18 0.38 0.01 0.005 1.07 0.0006 0.23 889 508 Comparison 2B 0.14 0.05 0.56 0.008 0.004 0.09 0.0014 0.65 857 477 Invention 2C 0.07 0.74 0.52 0.01 0.005 0.32 0.0027 0 889 514 Invention 2D 0.03 1.42 1.64 0.008 0.005 0.08 0.0011 0.28 947 476 Invention 2E 0.06 1.3 0.79 0.009 0.006 0.61 0.0013 0 918 516 Invention 2F 0.04 2.08 1.27 0.01 0.003 0.88 0.0016 0.18 968 512 Invention 2G 0.05 0.96 1.29 0.01 0.004 0.12 0.0035 0 908 485 Invention 2H 0.04 1.74 1.85 0.01 0.005 0.92 0.0017 0.24 955 490 Invention 2I 0.07 1.42 1.83 0.011 0.005 0.81 0.0014 0 920 478 Invention 2J 0.13 2.2 2.25 0.009 0.006 0.07 0.0033 0.46 950 415 Invention 2K 0.18 1.08 2.3 0.01 0.005 0.54 0.0025 0 872 412 Invention 2L 0.1 0.96 2.81 0.007 0.007 1.23 0.0026 0.34 899 440 Invention 2M 0.14 0.74 2.82 0.01 0.006 0.91 0.0044 0 867 417 Invention 2N 0.08 0.52 3.25 0.011 0.005 0.47 0.0023 0.65 896 405 Comparison 2O 0.03 1.44 1.36 0.012 0.006 0.006 0.0027 0.56 957 484 Comparison 2P 0.08 1.37 1.75 0.008 0.005 0.02 0.0015 0.47 929 451 Invention 2Q 0.12 1.36 2.41 0.008 0.005 0.02 0.0012 0 900 413 Invention 2R 0.15 2.33 1.52 0.01 0.005 0.98 0.0003 0.23 943 465 Invention 2S 0.04 0.31 1.98 0.01 0.005 0.12 0.002 0 883 462 Invention 2T 0.05 1.29 1.1 0.008 0.005 0.81 0.0011 0.78 947 509 Invention 2U 0.03 1.36 1.38 0.009 0.005 0.79 0.0015 0 936 509 Invention 2V 0.08 1.33 1.79 0.01 0.003 1.28 0.0008 0.05 914 489 Invention 2W 0.06 2.31 0.78 0.01 0.004 1.35 0.0004 0 964 538 Invention 2X 0.08 1.89 1.34 0.01 0.005 1.67 0.0017 0.16 942 518 Comparison 2Y 0.09 0.86 1.59 0.011 0.005 0.08 0.0016 0.03 888 458 Invention 2Z 0.12 1.87 1.73 0.01 0.005 0.45 0.0024 0.11 927 452 Invention

TABLE 3 Chemical component of specimen Chemical component (mass %) Steel code C Si Mn P S Al B Mo Other Ac3 (° C.) Ms point (° C.) Remarks 3A 0.07 2.12 2.64 0.008 0.007 0.09 0.0014 0.34 962 423 Invention 3B 0.17 1.19 1.04 0.008 0.005 0.73 0.0018 0.81 905 466 Invention 3C 0.05 1.07 1.08 0.007 0.006 0.77 0.0014 1.19 950 507 Comparison 3D 0.04 0.85 0.66 0.009 0.004 0.33 0 0.3 917 518 Comparison 3E 0.09 2.52 1.75 0.01 0.005 0.49 0.0001 0.12 966 463 Invention 3F 0.12 0.12 2.36 0.01 0.004 0.18 0.0001 0 845 420 Invention 3G 0.15 0.27 1.09 0.01 0.005 0.97 0.0005 0.81 869 478 Invention 3H 0.11 1.52 2.04 0.008 0.005 0.38 0.0005 0 911 442 Invention 3I 0.03 2.49 0.55 0.012 0.005 1.23 0.0011 0.34 997 553 Invention 3J 0.08 1.04 1.67 0.008 0.005 1.03 0.001 0 899 487 Invention 3K 0.05 1.82 1.78 0.011 0.006 0.49 0.0019 0.23 953 476 Invention 3L 0.09 0.26 2.68 0.01 0.004 0.05 0.0022 0 861 414 Invention 3M 0.11 1.28 1.73 0.009 0.005 0.08 0.0047 0.28 909 444 Invention 3N 0.04 1.82 1.88 0.01 0.005 0.58 0.0048 0 951 480 Invention 3O 0.19 1.45 0.89 0.011 0.005 0.69 0.0077 0.51 902 465 Comparison 3P 0.05 1.06 1.6 0.007 0.003 1.10 0.0012 0.77 Ti: 0.02 936 499 Invention 3Q 0.12 1.62 0.96 0.01 0.005 0.58 0.0009 0.36 Ti: 0.02 923 485 Invention 3R 0.08 2.41 1.26 0.01 0.005 0.22 0.0011 0.52 Ti: 0.08 977 476 Invention 3S 0.04 1.09 2.17 0.01 0.004 0.62 0.0004 0 Ti: 0.08 918 470 Invention 3T 0.09 0.46 0.97 0.008 0.006 0.08 0.0012 0.89 Ti: 0.12 898 478 Comparison 3U 0.12 1.66 1.78 0.009 0.005 0.07 0.0017 0.34 Nb: 0.02 925 438 Invention 3V 0.07 1.24 1.83 0.008 0.007 0.32 0.0005 0 Nb: 0.02 912 463 Invention 3W 0.17 0.73 2.14 0.011 0.006 1.04 0.0014 0.65 Nb: 0.08 879 433 Invention 3X 0.05 0.78 1.48 0.01 0.005 0.62 0.0019 0 Nb: 0.08 899 493 Invention 3Y 0.03 1.83 1.54 0.12 0.005 0.78 0.0021 0.49 Nb: 0.12 972 500 Comparison 3Z 0.08 2.22 1.92 0.01 0.004 0.93 0.0016 0.22 V: 0.02 961 472 Invention

TABLE 4 Chemical component of specimen Chemical component (mass %) Steel code C Si Mn P S Al B Mo Other Ac3 (° C.) Ms point (° C.) Remarks 4A 0.15 1.69 1.77 0.009 0.005 0.22 0.0034 0 V: 0.02 909 433 Invention 4B 0.11 2.3 0.92 0.01 0.003 0.79 0.0008 0.08 V: 0.08 956 495 Invention 4C 0.08 0.39 0.59 0.007 0.006 1.17 0.0017 0 V: 0.08 878 530 Invention 4D 0.07 1.71 1.18 0.01 0.005 0.98 0.0014 0.04 V: 0.12 946 504 Comparison 4E 0.05 1.96 0.67 0.01 0.005 0.82 0.0018 0.35 Ti: 0.02, Nb: 0.02 963 529 Invention 4F 0.09 2.07 2.45 0.01 0.004 0.05 0.0023 0 Ti: 0.02, Nb: 0.02 942 423 Invention 4G 0.06 1.37 0.72 0.008 0.007 0.09 0.0031 0.65 Ti: 0.03, V: 0.03 945 499 Invention 4H 0.04 1.22 0.92 0.008 0.006 0.84 0.0004 0 Ti: 0.03, V: 0.03 927 524 Invention 4I 0.11 1.08 2.15 0.008 0.005 0.48 0.0026 0.31 Nb: 0.04, V: 0.04 905 438 Invention 4J 0.13 0.11 2.4 0.011 0.005 0.08 0.0011 0 Nb: 0.04, V: 0.04 846 410 Invention 4K 0.08 1.47 0.86 0.008 0.005 0.33 0.0013 0.15 Ti: 0.02, Nb: 0.03 927 495 Invention V: 0.04 4L 0.05 1.57 1.39 0.009 0.005 0.38 0.0031 0 Ti: 0.02, Nb: 0.03 939 488 Invention V: 0.04 4M 0.03 1.24 1.54 0.012 0.005 0.81 0.0014 0.72 Ti: 0.05, Nb: 0.05 958 498 Comparison V: 0.05 4N 0.08 2.38 1.36 0.011 0.004 1.40 0.0019 0.37 Ni: 0.3 966 503 Invention 4O 0.11 1.29 1.08 0.01 0.005 0.38 0.0008 0 Ni: 0.3 896 474 Invention 4P 0.1 0.96 0.78 0.01 0.005 0.09 0.0034 0.79 Ni: 0.6 904 472 Comparison 4Q 0.09 2.24 0.94 0.011 0.003 0.03 0.0045 0.16 Cu: 0.3 954 478 Invention 4R 0.13 1.54 1.29 0.008 0.007 0.64 0.0029 0.47 Cu: 0.3 920 467 Invention 4S 0.18 0.73 2.06 0.007 0.005 0.11 0.0013 0.31 Cu: 0.6 866 400 Comparison 4T 0.03 2.08 1.63 0.011 0.005 0.43 0.0006 0.56 Ni: 0.2, Cu: 0.2 982 484 Invention 4U 0.1 1.39 0.87 0.01 0.005 0.63 0.0003 0 Ni: 0.2, Cu: 0.2 905 497 Invention

TABLE 5 Chemical component of specimen Steel Chemical component (mass %) Ac3 Ms point code C Si Mn P S Al B Mo Other (° C.) (° C.) Remarks 4V 0.08 0.54 1.45 0.01 0.004 1.24 0.0006 0.68 Cr: 0.2 898 494 Invention 4W 0.05 1.87 1.9 0.008 0.005 0.45 0.0018 0 Cr: 0.2 948 467 Invention 4X 0.06 0.94 1.44 0.011 0.006 0.98 0.0031 0.22 Cr: 0.5 909 490 Invention 4Y 0.09 1.73 1.51 0.008 0.005 1.19 0.0023 0 Cr: 0.5 926 484 Invention 4Z 0.08 1.23 1.63 0.01 0.005 0.97 0.0017 0.75 Cr: 0.8 931 467 Invention 5A 0.18 0.57 0.9 0.01 0.005 0.66 0.0022 0 Cr: 0.8 849 454 Invention 5B 0.16 1.11 1.27 0.012 0.005 1.29 0.0034 0.36 Cr: 1.6 890 448 Comparison 5C 0.07 2.05 2.88 0.01 0.003 0.18 0.002 0.09 Ca: 0.002 951 417 Invention 5D 0.08 1.06 1.5 0.009 0.005 0.11 0.0005 0 Ca: 0.002 900 466 Invention 5E 0.04 1.43 1.27 0.008 0.004 0.30 0.004 0.26 REM: 0.002 942 496 Invention 5F 0.13 0.92 1.66 0.01 0.006 0.52 0.0013 0.05 REM: 0.002 880 454 Invention

TABLE 6 Heat treatment condition Heat treatment condition Cooling Heating Cooling stop Material Steel temperature rate temperature No. code (° C.) (° C./sec) (° C.) Remarks 1 1A 1000 2 10 Comparison 2 1B 1000 2 10 Invention 3 1C 1000 2 10 Invention 4 1D 800 2 10 Comparison 5 1D 850 2 10 Comparison 6 1D 900 2 10 Comparison 7 1D 1000 2 10 Invention 8 1D 1100 2 10 Invention 9 1D 1200 2 10 Invention 10 1E 1000 2 10 Invention 11 1F 1000 2 10 Invention 12 1G 1000 2 10 Invention 13 1H 1000 2 10 Invention 14 1I 1000 2 10 Invention 15 1J 1000 2 10 Invention 16 1K 1000 2 10 Invention 18 1M 1000 2 10 Comparison 19 1N 1000 2 10 Invention 20 1O 1000 2 10 Invention 21 1P 1000 2 10 Invention 22 1Q 1000 2 10 Invention 23 1R 1000 0.05 10 Comparison 24 1R 1000 0.1 10 Comparison 25 1R 1000 0.3 10 Invention 26 1R 1000 0.5 10 Invention 27 1R 1000 1 10 Invention 28 1R 1000 5 10 Invention 29 1R 1000 10 10 Invention 30 1R 1000 20 10 Invention 31 1R 1000 30 10 Comparison 32 1R 1000 50 10 Comparison 33 1R 1000 100 10 Comparison 34 1R 1000 500 10 Comparison 35 1S 1000 0.05 10 Comparison

TABLE 7 Heat treatment condition Heat treatment condition Cooling Heating Cooling stop Material Steel temperature rate temperature No. code (° C.) (° C./sec) (° C.) Remarks 36 1S 1000 0.1 10 Comparison 37 1S 1000 0.3 10 Invention 38 1S 1000 0.5 10 Invention 39 1S 1000 1 10 Invention 40 1S 1000 5 10 Invention 41 1S 1000 10 10 Invention 42 1S 1000 20 10 Invention 43 1S 1000 30 10 Comparison 44 1S 1000 50 10 Comparison 45 1S 1000 100 10 Comparison 46 1S 1000 500 10 Comparison 47 1T 1000 2 10 Invention 48 1U 1000 2 10 Invention 49 1V 1000 2 10 Invention 50 1W 1000 2 10 Invention 51 1X 1000 2 10 Invention 52 1Y 1000 2 10 Invention 53 1Z 1000 2 10 Comparison 54 2A 1000 2 10 Comparison 55 2B 1000 2 10 Invention 56 2C 1000 2 10 Invention 57 2D 1000 2 20 Invention 58 2D 1000 2 100 Invention 59 2D 1000 2 300 Invention 60 2D 1000 2 500 Comparison 61 2E 1000 2 10 Invention 62 2F 1000 2 10 Invention 63 2G 1000 2 10 Invention 64 2H 1000 2 10 Invention 65 2I 1000 2 10 Invention 66 2J 1000 2 10 Invention 67 2K 1000 2 10 Invention 68 2L 1000 2 10 Invention 69 2M 1000 2 10 Invention 70 2N 1000 2 10 Comparison

TABLE 8 Heat treatment condition Heat treatment condition Cooling Heating Cooling stop Material Steel temperature rate temperature No. code (° C.) (° C./sec) (° C.) Remarks 71 2O 1000 2 10 Comparison 72 2P 1000 2 10 Invention 73 2Q 1000 2 10 Invention 74 2R 1000 2 10 Invention 75 2S 1000 2 10 Invention 76 2T 1000 2 10 Invention 77 2U 1000 2 10 Invention 78 2V 1000 2 10 Invention 79 2W 1000 2 10 Invention 80 2X 1000 2 10 Comparison 81 2Y 1000 2 10 Invention 82 2Z 1000 2 10 Invention 83 3A 1000 2 10 Invention 84 3B 1000 2 10 Invention 85 3C 1000 2 10 Comparison 86 3D 1000 2 10 Comparison 87 3E 1000 2 10 Invention 88 3F 1000 2 10 Invention 89 3G 1000 2 10 Invention 90 3H 1000 2 10 Invention 91 3I 1000 2 10 Invention 92 3J 1000 2 10 Invention 93 3K 1000 2 10 Invention 94 3L 1000 2 10 Invention 95 3M 1000 2 10 Invention 96 3N 1000 2 10 Invention 97 3O 1000 2 10 Comparison 98 3P 1000 2 10 Invention 99 3Q 1000 2 10 Invention 100 3R 1000 2 10 Invention 101 3S 1000 2 10 Invention 102 3T 1000 2 10 Comparison 103 3U 1000 2 10 Invention 104 3V 1000 2 10 Invention 105 3W 1000 2 10 Invention

TABLE 9 Heat treatment condition Heat treatment condition Cooling Heating Cooling stop Material Steel temperature rate temperature No. code (° C.) (° C./sec) (° C.) Remarks 106 3X 1000 2 10 Invention 107 3Y 1000 2 10 Comparison 108 3Z 1000 2 10 Invention 109 4A 1000 2 10 Invention 110 4B 1000 2 10 Invention 111 4C 1000 2 10 Invention 112 4D 1000 2 10 Comparison 113 4E 1000 2 10 Invention 114 4F 1000 2 10 Invention 115 4G 1000 2 10 Invention 116 4H 1000 2 10 Invention 117 4I 1000 2 10 Invention 118 4J 1000 2 10 Invention 119 4K 1000 2 10 Invention 120 4L 1000 2 10 Invention 121 4M 1000 2 10 Comparison 122 4N 1000 2 10 Invention 123 4O 1000 2 10 Invention 124 4P 1000 2 10 Comparison 125 4Q 1000 2 10 Invention 126 4R 1000 2 10 Invention 127 4S 1000 2 10 Comparison 128 4T 1000 2 10 Invention 129 4U 1000 2 10 Invention 130 4V 1000 2 10 Invention 131 4W 1000 2 10 Invention 132 4X 1000 2 10 Invention 133 4Y 1000 2 10 Invention 134 4Z 1000 2 10 Invention 135 5A 1000 2 10 Invention 136 5B 1000 2 10 Comparison 137 5C 1000 2 10 Invention 138 5D 1000 2 10 Invention 139 5E 1000 2 10 Invention 140 5F 1000 2 10 Invention

Microstructure and dynamic properties were examined on each of steel sheets obtained above by the following procedures.

Microstructure of each steel was determined by the following method.

Each of steel sheet Nos. 1 to 140 after heat treatment was cut into a test piece for microstructure observation of 10 mm×10 mm×1.2 mm. The test piece was embedded with a cold rolled resin such that the observation position is a position of ¼ of a sheet thickness in a rolled direction. Impression as a landmark was formed with a Vickers tester so as to conduct identification on the structure observation place. The impression was corroded with repeller, and the structure was observed at five positions using an optical microscope in a magnification of 1,000. When the structure photograph after repeller corrosion was image analyzed, ferrite is observed gray, and martensite and retained austenite were observed white. After microstructure observation with an optical microscope, buff polishing and electrolysis polishing were conducted to an extent such that the Vickers impression does not disappear, and structure observation was conducted on the place at a step distance of 100 nm using FE/SEM-EBSP. A boundary having an orientation difference of 15° or more of grains was considered as grain boundary, and the grain boundary was identified.

Optical microphotograph and FE/SEM-EBSP structure photograph were combined as the standard of Vickers impression, and the structure was evaluation.

MD Structure:

The MD structure contains fine martensite (secondary phase) in ferrite grains as a matrix, as described above. The observation results of optical microscope and FE/SEM-EBSP were combined, a structure in which many martensite are present in the inside of grains surrounded by large tilt grain boundary (i.e., in the grains) was identified as MD structure, and its space factor was obtained.

There is the possibility that pearlite, bainite, retained austenite, ferrite which is not accompanied with martensite in grains, and the like are contained as other structures. Identification method of those structures is as follow.

Retained Austenite:

Retained austenite has an FCC structure, and therefore can be identified with FE/SEM-EBSP. Area fraction of austenite in the observation view was determined.

Pearlite:

Pearlite has a lamella structure of ferrite and cementite, and the cementite can be identified with FE/SEM-EBSP. In the Examples, the area fraction of pearlite in the observation view was determined.

Bainite:

Bainite is that lath is present in ferrite grains identified with FE/SEM-EBSP. Structure in which lath is present in ferrite grains was judged as bainite, and area fraction of bainite in the observation view was determined.

Ferrite which is not Accompanied with Martensite in Grains:

Structure in which martensite cannot be confirmed in ferrite grains identified with FE/SEM-EBSP is considered as ferrite which is not accompanied with martensite in grains, and is judged as a structure different from MD structure. Area fraction of ferrite which is not accompanied with martensite in grains in the observation view was determined.

Structure in which MD structure is not observed in the observation view is judged that MD is not present. When the whole structure is defined as 100%, in the case that the total of area fractions of other structures (retained austenite, pearlite, bainite, ferrite which is not accompanied with martensite in grains, and the like) exceeds 10%, the defined structure is not formed. In this case, it was judged that MD is partial.

On the other hand, in the case that the space factor of MD structure occupies 90% or more of the whole structure, the following further detailed investigation was conducted.

Space Factor and Average Grain Size of Martensite:

Space factor of martensite in the whole structure used area fraction of white portion in the whole structure by image analyzing an optical microphotograph. By combining with an image of FE/SEM-EBSP, martensite present in ferrite grains and martensite present at grain boundary were separated. Retained austenite is also observed as a white portion, but austenite can be distinguished from retained austenite by identifying retained austenite which has an FCC structure, with FE/SEM-EBSP. Retained austenite was removed by image analysis of FE/SEM-EBSP, and the space factor of martensite was determined.

On the other hand, an average grain size (particle diameter) of martensite was determined by the following method. Five places of the observation view having a size of 20 μm×20 μm were randomly extracted from a SEM observation photograph (magnification 3,000 times), an average grain size (diameter of a corresponding circle) of martensite and retained austenite in the respective observation views was determined, and its average value was used as a grain size of martensite in the whole structure.

Dynamic properties of each steel were determined by the following methods.

Tensile Properties:

JIS No. 5 test piece obtained from a vertical direction of a steel sheet rolled direction was used, and tensile strength (TS) and Elongation (EL) were measured according to JIS Z 2241. Test pieces having TS of 590 MPa or more and EL of 10% or more were considered as acceptable products. Elongation from strain corresponding to tensile strength to breaking strain was designated as local elongation.

Stretch Flangeability:

Hole expanding ratio λ was measured as stretch flangeability. The hole expanding ratio λ was measured according to JISF (The Japan Iron and Steel Federation) standard (JFST 1001-1996). Steels having λ of 80% or more were considered as acceptable products.

The results obtained are shown in Tables 10 to 15 below.

TABLE 10 Measurement results Structure parameter Af of α′ Grain size Af of α′ in α grain of α′ in Dynamic properties Presence Af of MD in entire of entire entire Total Local Material Steel or absence structure structure structure structure TS elongation elongation λ No. code of MD (%) (%) (%) (μm) (MPa) (%) (%) (%) Remarks 1 1A 94.5 18.8 70.9 1.5 532 32.9 25.1 169.2 Comparison 2 1B 95.9 27.2 71.9 1.7 668 27.6 19.8 136.8 Invention 3 1C 95.0 45.5 71.3 1.7 933 18.8 10.2 96.5 Invention 4 1D Partial 28.1 687 18.3 6.5 39.5 Comparison 5 1D Partial 50.1 715 17.5 6.7 45.2 Comparison 6 1D Partial 87.4 766 16.4 6.3 43.3 Comparison 7 1D 95.4 42.5 77.6 1.6 854 17.2 10.5 105.8 Invention 8 1D 94.1 43.6 77.7 1.6 832 18.4 11.4 108.5 Invention 9 1D 94.1 41.3 78.8 1.7 844 19.4 12.3 110.1 Invention 10 1E 90.4 31.9 67.8 1.9 729 24.0 18.0 123.4 Invention 11 1F 91.1 29.7 68.3 1.5 696 25.1 18.9 129.3 Invention 12 1G 91.1 39.3 68.3 2.1 849 21.9 14.1 107.3 Invention 13 1H 96.5 58.6 72.4 2.3 1289 14.7 6.2 80.3 Invention 14 1I 94.8 57.4 71.1 2.3 1189 14.7 6.3 85.7 Invention 15 1J 93.6 45.9 70.2 2.6 949 19.6 11.6 95.9 Invention 16 1K 92.3 57.9 69.2 2.7 1119 15.6 9.7 80.4 Invention 18 1M None 843 9.2 4.4 67.4 Comparison 19 1N 94.8 36.7 71.1 1.8 801 21.9 13.5 112.4 20 1O 92.4 23.2 69.3 1.7 597 29.3 22.4 150.7 Invention 21 1P 93.9 30.1 70.4 2.0 711 26.0 17.2 128.3 Invention 22 1Q 91.2 31.6 68.4 2.3 724 24.2 17.4 124.3 Invention 23 1R None 641 26.2 10.5 41.5 Comparison α: Ferrite, α′: Martensite, Af: Area fraction, TS: Tensile strength, λ: Hole expanding ratio

TABLE 11 Measurement results Structure parameter Af of α′ Grain size Af of α′ in α grain of α′ in Dynamic properties Presence Af of MD in entire of entire entire Total Local Material Steel or absence structure structure structure structure TS elongation elongation λ No. code of MD (%) (%) (%) (μm) (MPa) (%) (%) (%) Remarks 24 1R Partial 72.9 732 18.1 7.1 52.3 Comparison 25 1R 96.1 38.0 78.4 1.5 867 19.4 14.8 102.5 Invention 26 1R 96.7 39.3 77.4 1.7 856 20.5 14.9 112.9 Invention 27 1R 96.4 40.9 78.6 1.4 876 20.2 15.9 108.7 Invention 28 1R 96.3 38.5 80.0 1.4 873 19.2 15.7 112.4 Invention 29 1R 96.0 38.2 78.4 1.5 884 20.9 15.9 115.3 Invention 30 1R 95.4 38.9 80.3 1.6 871 20.8 16.2 105.8 Invention 31 1R None 1021 9.2 4.7 74.3 Comparison 32 1R None 1101 8.4 4.1 72.1 Comparison 33 1R None 1209 8.1 3.6 79.4 Comparison 34 1R None 1234 7.5 3.8 76.1 Comparison 35 1S None 611 25.9 10.5 38.5 Comparison 36 1S Partial 62.8 722 17.3 6.9 45.9 Comparison 37 1S 95.1 37.6 77.6 1.7 856 20.4 15.6 100.5 Invention 38 1S 95.7 38.9 76.7 1.9 844 20.9 14.7 109.2 Invention 39 1S 95.4 40.5 77.8 1.6 860 19.7 16.1 106.5 Invention 40 1S 95.3 38.1 79.2 1.6 861 18.7 15.5 110.9 Invention 41 1S 95.0 37.8 77.6 1.7 863 21.3 15.7 112.3 Invention 42 1S 94.5 38.5 79.5 1.8 859 20.5 16.0 101.6 Invention 43 1S None 989 9.5 4.5 72.1 Comparison 44 1S None 1088 82 3.9 70.9 Comparison 45 1S None 1173 7.9 3.4 76.5 Comparison 46 1S None 1198 7.2 3.5 75.5 Comparison α: Ferrite, α′: Martensite, Af: Area fraction, TS: Tensile strength, λ: Hole expanding ratio

TABLE 12 Measurement results Structure parameter Af of α′ Grain size Af of α′ in α grain of α′ in Dynamic properties Presence Af of MD in entire of entire entire Total Local Material Steel or absence structure structure structure structure TS elongation elongation λ No. code of MD (%) (%) (%) (μm) (MPa) (%) (%) (%) Remarks 47 1T 92.1 39.6 69.0 2.0 854 21.7 13.5 106.6 Invention 48 1U 95.0 64.8 71.2 2.6 1221 14.3 5.8 87.7 Invention 49 1V 93.4 48.4 70.1 1.9 986 18.9 11.4 92.2 Invention 50 1W 93.6 63.4 70.2 2.3 1201 14.6 6.7 84.9 Invention 51 1X 97.0 62.3 72.7 1.7 1184 14.8 7.5 96.0 Invention 52 1Y 93.3 68.1 70.0 2.6 1271 13.8 6.0 90.8 Invention 53 1Z 90.1 66.3 67.5 2.0 1245 9.1 3.0 72.3 Comparison 54 2A Partial 72.5 661 26.5 20.9 54.7 Comparison 55 2B 94.5 25.7 70.8 2.4 635 27.6 21.7 141.8 Invention 56 2C 90.9 24.2 68.2 1.9 623 29.5 22.7 146.7 Invention 57 2D 94.1 45.1 75.6 1.7 865 18.7 13.2 100.4 Invention 58 2D 94.3 44.5 78.9 1.8 878 18.6 13.3 108.1 Invention 59 2D 94.5 48.1 77.5 1.8 855 17.9 12.8 107.3 Invention 60 2D None 954 9.8 4.8 79.8 Comparison 61 2E 91.2 28.4 68.4 1.9 676 25.9 19.7 133.2 Invention 62 2F 92.9 46.2 69.7 1.7 953 19.6 11.7 95.4 Invention 63 2G 91.5 29.0 68.6 1.8 695 26.6 18.6 131.4 Invention 64 2H 95.4 51.9 71.5 1.7 1038 18.0 9.2 87.5 Invention 65 2I 93.3 49.9 70.0 1.9 1009 18.5 9.8 90.1 Invention 66 2J 98.5 53.3 73.9 2.3 1200 14.6 6.2 85.0 Invention 67 2K 94.9 54.0 71.1 2.6 1210 14.5 6.0 84.4 Invention 68 2L 95.4 55.6 71.6 2.1 1100 17.1 6.8 82.6 Invention 69 2M 96.6 58.6 72.4 2.4 1129 15.5 6.3 89.7 Invention α: Ferrite, α′: Martensite, Af: Area fraction, TS: Tensile strength, λ: Hole expanding ratio

TABLE 13 Measurement results Structure parameter Af of α′ Grain size Af of α′ in α grain of α′ in Dynamic properties Presence Af of MD in entire of entire entire Total Local Material Steel or absence structure structure structure structure TS elongation elongation λ No. code of MD (%) (%) (%) (μm) (MPa) (%) (%) (%) Remarks 70 2N 98.4 55.4 73.8 2.0 1242 15.2 4.3 73.1 Comparison 71 2O 96.4 35.1 72.3 1.7 787 9.5 3.5 65.8 Comparison 72 2P 96.9 47.5 72.7 2.0 972 19.2 10.6 93.5 Invention 73 2Q 95.2 56.7 71.4 2.3 1174 14.9 6.3 86.6 Invention 74 2R 94.2 58.5 70.6 2.4 1128 15.5 8.2 89.8 Invention 75 2S 93.8 28.8 70.3 1.7 691 26.7 17.7 132.1 Invention 76 2T 97.3 37.6 73.0 1.8 824 22.5 14.9 110.6 Invention 77 2U 91.8 35.1 68.8 1.7 776 22.5 15.5 116.0 Invention 78 2V 93.6 52.7 70.2 2.0 1050 17.8 9.1 86.6 Invention 79 2W 92.7 45.6 69.5 1.9 933 18.7 12.6 96.4 Invention 80 2X 94.8 66.7 71.1 3.8 1100 7.9 4.5 81.8 Comparison 81 2Y 92.7 37.6 69.6 2.1 824 22.5 14.1 110.6 Invention 82 2Z 93.9 56.1 70.4 2.3 1186 15.9 7.3 86.5 Invention 83 3A 96.8 56.4 72.6 1.9 1106 17.0 7.0 82.2 Invention 84 3B 97.4 55.6 73.0 2.6 1084 16.1 9.6 83.0 Invention 85 3C Partial 88.7 803 24.8 18.2 72.1 Comparison 86 3D Partial 41.9 589 29.7 23.7 52.7 Comparison 87 3E 94.0 57.3 70.5 2.1 1120 16.8 8.1 81.1 Invention 88 3F 95.1 44.7 71.3 2.3 920 19.0 10.5 97.8 Invention 89 3G 97.6 41.5 73.2 2.4 873 20.1 13.4 103.1 Invention 90 3H 94.0 57.8 70.5 2.2 1117 15.7 7.6 80.6 Invention 91 3I 91.9 43.7 68.9 1.7 905 19.3 13.5 99.4 Invention 92 3J 92.8 45.1 69.6 2.0 937 19.9 11.4 97.1 Invention α: Ferrite, α′: Martensite, Af: Area fraction, TS: Tensile strength, λ: Hole expanding ratio

TABLE 14 Measurement results Structure parameter Af of α′ Grain size Af of α′ in α grain of α′ in Dynamic properties Presence Af of MD in entire of entire entire Total Local Material Steel or absence structure structure structure structure TS elongation elongation λ No. code of MD (%) (%) (%) (μm) (MPa) (%) (%) (%) Remarks 93 3K 95.0 50.8 71.3 1.8 1022 18.3 9.6 89.0 Invention 94 3L 96.1 47.6 72.1 2.1 963 18.2 9.1 93.4 Invention 95 3M 95.3 52.9 71.5 2.2 1053 17.8 9.2 86.3 Invention 96 3N 93.5 52.3 70.1 1.7 1044 17.9 9.1 87.0 Invention 97 3O 94.4 62.7 70.8 2.7 1191 14.7 8.4 75.6 Comparison 98 3P 98.9 44.6 74.2 1.8 929 20.0 11.6 97.9 Invention 99 3Q 93.4 47.1 70.0 2.3 956 18.3 11.9 94.1 Invention 100 3R 95.7 54.5 71.8 2.0 1077 17.4 9.5 84.4 Invention 101 3S 94.4 44.0 70.8 1.7 910 19.2 11.0 98.9 Invention 102 3T 97.8 27.3 73.4 2.1 659 26.6 20.1 66.6 Comparison 103 3U 95.9 58.2 72.0 2.3 1132 16.6 7.9 80.2 Invention 104 3V 93.3 43.6 70.0 1.9 914 20.4 11.6 99.5 Invention 105 3W 96.7 58.2 72.5 2.6 1273 13.7 5.5 86.7 Invention 106 3X 92.1 31.7 69.1 1.8 735 25.1 16.9 124.2 Invention 107 3Y 96.4 47.3 72.3 1.7 960 18.2 10.9 73.8 Comparison 108 3Z 95.4 55.9 71.6 2.0 1248 15.1 6.3 82.7 Invention 109 4A 93.1 56.7 69.8 2.4 1205 14.5 6.9 84.7 Invention 110 4B 90.9 53.2 68.2 2.2 1049 16.7 10.3 85.8 Invention 111 4C 91.2 27.2 68.4 2.0 658 26.6 20.7 136.9 Invention 112 4D 91.5 44.5 68.6 1.9 928 20.1 12.3 68.1 Comparison 113 4E 92.3 37.4 69.3 1.8 820 22.6 15.6 111.0 Invention 114 4F 95.4 54.6 71.5 2.1 1165 15.0 6.3 83.2 Invention 115 4G 95.0 30.3 71.2 1.9 704 24.9 18.8 127.8 Invention 116 4H 90.3 26.8 67.7 1.7 653 26.8 20.4 137.9 Invention α: Ferrite, α′: Martensite, Af: Area fraction, TS: Tensile strength, λ: Hole expanding ratio

TABLE 15 Measurement results Structure parameter Af of α′ Grain size Af of α′ in α grain of α′ in Dynamic properties Presence Af of MD in entire of entire entire Total Local Material Steel or absence structure structure structure structure TS elongation elongation λ No. code of MD (%) (%) (%) (μm) (MPa) (%) (%) (%) Remarks 117 4I 96.9 58.1 72.7 2.2 1132 16.6 7.4 80.2 Invention 118 4J 95.2 47.0 71.4 2.3 955 18.3 9.7 94.2 Invention 119 4K 91.3 34.2 68.5 2.0 773 23.9 16.7 118.0 Invention 120 4L 91.8 40.0 68.9 1.8 860 21.6 13.5 105.9 Invention 121 4M 98.3 40.6 73.7 1.7 859 20.4 13.1 64.8 Comparison 122 4N 94.8 57.1 71.1 2.0 1207 15.6 7.6 85.2 Invention 123 4O 90.8 39.4 68.1 2.2 851 21.8 14.2 107.0 Invention 124 4P 96.4 33.7 72.3 2.1 765 24.2 17.0 59.2 Comparison 125 4Q 91.7 48.0 68.7 2.1 971 18.0 11.6 92.7 Invention 126 4R 95.4 55.8 71.5 2.3 1097 17.1 9.2 82.8 Invention 127 4S 96.6 56.9 72.5 2.6 1164 16.2 7.1 78.0 Comparison 128 4T 97.3 49.0 73.0 1.7 995 18.8 10.3 91.4 Invention 129 4U 90.1 36.9 67.6 2.1 803 21.8 15.5 112.1 Invention 130 4V 97.7 40.2 73.3 2.0 863 21.5 13.3 105.5 Invention 131 4W 93.5 51.6 70.1 1.8 1034 18.1 9.2 87.9 Invention 132 4X 93.8 39.5 70.4 1.9 852 21.8 13.6 106.8 Invention 133 4Y 92.2 55.4 69.2 2.1 1091 17.2 8.9 83.3 Invention 134 4Z 98.9 51.8 74.1 2.0 1037 18.0 9.6 87.6 Invention 135 5A 90.2 41.7 67.6 2.6 876 20.0 13.6 102.7 Invention 136 5B 94.4 58.8 70.8 2.5 1133 7.5 4.8 79.5 Comparison 137 5C 97.5 58.9 73.2 1.9 1133 15.4 6.1 88.4 Invention 138 5D 92.2 36.2 69.1 2.0 803 23.1 14.8 113.5 Invention 139 5E 93.6 36.4 70.2 1.7 805 23.0 15.1 113.2 Invention 140 5F 93.1 49.0 69.9 2.3 985 17.8 10.3 91.4 Invention α: Ferrite, α′: Martensite, Af: Area fraction, TS: Tensile strength, λ: Hole expanding ratio

From the results shown in Tables 10 to 15, it can be considered as follows. The following alphabets all mean steel codes in Tables 1 to 15, and the following Nos. all mean material Nos. in Tables 6 to 15. Nos. 2 to 3, 7 to 16, 19 to 22, 25 to 30, 37 to 42, 47 to 52, 55 to 59, 61 to 69, 72 to 79, 81 to 84, 87 to 96, 98 to 101, 103 to 106, 108 to 111, 113 to 120, 122 to 123, 125 to 126, 128 to 135 and 137 to 140 all are examples that high strength steel sheets having the structure defined in the present invention were produced by the production method defined in the present invention using steel species satisfying the scope of the present invention (1B to 1K, 1N to 1Y, 2B to 2M, 2P to 2W, 2Y to 3B, 3E to 3N, 3P to 3S, 3U to 3X, 3Z to 4C, 4E to 4L, 4N to 4O, 4Q to 4R, 4T to 5A and 5C to 5F in Tables 1 to 5). The high strength steel sheets shown by the above Nos. all have excellent tensile strength, elongation and stretch flangeability.

Contrary to this, the following examples that are not satisfied with any one of the requirements specified in the present invention have the following disadvantages.

No. 1 is an example using steel code 1A having a small C content. Because the proportion of martensite in the whole structure is lower than the range defined, tensile strength was decreased.

Nos. 4 to 6 each use steel code 1D having a component composition which is satisfied with the provision of the present invention. However, the heating and holding temperature of the heat treatment is not A3 point or higher, and the area fraction of α+α′ is less than 90%. Therefore, stretch flangeability was decreased.

No. 18 is an example using steel code 1M having a small Si content. Because it has a bainite structure over the entire surface and MD is not present, elongation is low.

Nos. 23 to 24 each use steel code 1R having a component composition which is satisfied with the provision of the present invention, and Nos. 35 to 36 each use steel code 1S having a component composition which is satisfied with the provision of the present invention. However, the cooling rate of the heat treatment is slower than the range defined, and as a result, ferrite which is not accompanied with martensite in grains was formed. No. 23 and No. 35 each have a ferrite-pearlite structure, and MD is not present. No. 24 and No. 36 each are that ferrite which is not accompanied with martensite in grains is formed and the area fraction of α+α′ is less than 90%. Therefore, stretch flangeability is low.

Nos. 31 to 34 each use steel code 1R having a component composition which is satisfied with the provision of the present invention, and Nos. 43 to 46 each use steel code 1S having a component composition which is satisfied with the provision of the present invention. However, the cooling rate of the heat treatment is slower than the range defined. Therefore, Nos. 31 and 32 and Nos. 43 and 44 are that lath of bainite does not completely disappear, thereby a bainitic ferrite-martensite structure is formed and MD is not present. As a result, elongation and stretch flangeability were decreased. Furthermore, No. 33 and 34 and Nos. 45 and 46 each are that a martensite single phase structure is formed, and elongation and stretch flangeability were decreased. No. 53 is an example using steel code 1Z having a large Si content. Because Si was added in a large amount, elongation and stretch flangeability were decreased.

No. 54 is an example using steel code 2A having a small Mn content.

Because pearlite was formed and α+α′ was less than 90%, stretch flangeability is low. No. 60 uses steel code 2D which is satisfied with the provision of the present invention. However, because the cooling stop temperature exceeds Ms point, a bainite structure was formed on the entire surface, and elongation and stretch flangeability were decreased.

No. 70 is an example using steel code 2N having a large Mn content, and stretch flangeability was decreased.

No. 71 is an example using steel code 2O having a small Al content.

Compounds of Si, Mn and O were unavoidably formed, and elongation and stretch flangeability were decreased.

No. 80 is an example using steel code 2X having a large Al content. Course martensite was formed in a large amount, and the area fraction of α′ in the whole structure and an average gain size of α′ exceeded the defined ranges. As a result, elongation and stretch flangeability was decreased.

No. 85 is an example using steel code 3C having a large Mo content. Because course retained austenite was formed in a large amount and α+α′ was less than 90%, stretch flangeability was decreased.

No. 86 is an example using steel code 3D having a small B content. Because ferrite which is not accompanied with martensite in grains was precipitated and α+α′ was less than 90%, stretch flangeability was decreased.

No. 97 is an example using steel code 3O having a large B content. Stretch flangeability was decreased due to segregation of B.

No. 102 is an example using steel code 3T having a large Ti content. Because precipitate was increased, stretch flangeability was decreased. No. 107 is an example using steel code 3Y having a large Nb content. Because precipitate was increased, stretch flangeability was decreased. No. 112 is an example using steel code 4D having a large V content. Because precipitate was increased, stretch flangeability was decreased. No. 121 is an example using steel code 4M having a large total addition amount of Ti, Nb and V Because precipitate was increased, stretch flangeability was decreased.

No. 124 is an example using steel code 4P having a large Ni content. Because Ni was added in a large amount, it is liable to break and stretch flangeability was decreased.

No. 127 is an example using steel code 4S having a large Cu content. Because Cu was added in a large amount, it is liable to break and stretch flangeability was decreased.

No. 136 is an example using steel code 5B having a large Cr content. Because Cr was added in a large amount, it is liable to break and stretch flangeability was decreased.

Next, examples in the case that the secondary phase is martensite and retained austenite are described below.

Steels A to W having component compositions shown in Table 16 were melted to form slabs. The slabs were heated to 1,200° C., hot rolled to a sheet thickness of 3.0 mm at 800° C. and then taken up at 550° C. The thermo-mechanical treatment (heating temperature T1 (° C.), processing temperature T2 (° C.), rolling reduction P (%), cooling rate R (° C./sec)) shown in FIG. 1 and Table 17 was conducted to cool to room temperature, and thickness reduction processing was conducted so as to be a sheet thickness of 1.2 mm.

TABLE 16 Chemical component of specimen Steel Chemical component (mass %) Ac3 code C Si Mn P S Al Mo B Other (° C.) Remarks A 0.05 1.4 1.3 0.01 0.005 0.05 0.3 0.0008 Cr: 0.65, Ti: 0.015 923 Invention B 0.03 1.3 1.4 0.01 0.005 0.05 0.3 0.0007 927 Invention C 0.08 1.4 1.4 0.01 0.005 0.03 0.3 0.0007 902 Invention D 0.25 1.5 1.5 0.01 0.005 0.05 0 0.0009 858 Invention E 0.35 1.7 2.5 0.01 0.005 0.05 0.3 0.0005 827 Comparison F 0.12 0.008 1.4 0.01 0.005 0.05 0.3 0.0005 834 Comparison G 0.10 0.03 1.4 0.01 0.005 1.1 0.3 0.0008 1262 Invention H 0.05 0.28 0.7 0.01 0.005 0.05 0.3 0.0003 893 Invention I 0.07 3.6 2.5 0.01 0.005 0.05 0.3 0.0008 979 Comparison J 0.05 0.4 0.3 0.01 0.005 0.06 0 0.0005 904 Comparison K 0.09 1.5 3.6 0.01 0.005 0.05 0.3 0.0008 845 Comparison L 0.05 0.03 2.2 0.01 0.005 0.005 0.3 0.0008 818 Comparison M 0.05 1.5 1.5 0.01 0.005 0.05 0 0.001 914 Invention N 0.05 1.4 1.4 0.01 0.005 0.05 0.7 0.0006 934 Invention O 0.07 1.2 2.2 0.01 0.005 0.05 1.3 0.0008 912 Comparison P 0.05 1.2 0.9 0.01 0.005 0.05 0.1 0 921 Comparison Q 0.05 1.5 1.4 0.01 0.005 0.05 0 0.0035 917 Invention R 0.05 1.5 1.3 0.01 0.005 0.05 0.3 0.007 929 Comparison S 0.10 1.4 1.3 0.01 0.005 0.05 0.3 0.0008 Ti: 0.015, Nb: 0.012 912 Invention T 0.05 1.4 1.3 0.01 0.005 0.05 0.3 0.0008 V: 0.0012 926 Invention U 0.05 1.4 1.3 0.01 0.005 0.05 0.3 0.0008 Ni: 0.1, Cu: 0.1 921 Invention V 0.07 1.2 1.2 0.01 0.005 0.05 0.3 0.0008 Ca: 0.002, REM: 0.002 910 Invention W 0.01 0.5 1.2 0.01 0.005 0.05 0.3 0.0008 913 Comparison X 0.01 0.3 1.8 0.01 0.005 0.05 0.3 0.0007 886 Comparison

TABLE 17 Heat treatment condition Heat treatment condition Heating Heating Cooling temperature temperature Rolling temperature Material Steel T1 T2 reduction P R No. code (° C.) (° C.) (%) (° C.) Remarks 141 A 1200 900 30 10 Invention 142 B 1200 900 30 10 Invention 143 C 1250 800 30 10 Invention 144 D 1200 800 20 5 Invention 145 E 1200 950 10 18 Comparison 146 F 1200 750 40 7 Comparison 147 G 1300 700 50 10 Invention 148 H 1250 800 30 10 Invention 149 I 1200 800 30 15 Comparison 150 J 1250 700 40 0.5 Comparison 151 K 1200 900 20 15 Comparison 152 L 1200 900 30 10 Comparison 153 M 1250 800 30 10 Invention 154 N 1200 650 30 0.5 Invention 155 O 1200 900 10 15 Comparison 156 P 1200 700 30 0.5 Comparison 157 Q 1200 750 30 0.5 Invention 158 R 1200 900 10 15 Comparison 159 S 1300 700 30 10 Invention 160 T 1300 700 40 10 Invention 161 U 1200 800 30 10 Invention 162 V 1200 700 30 10 Invention 163 W 1200 700 30 0.5 Comparison 164 C 1200 550 30 1 Comparison 165 C 1300 1050 10 0.5 Comparison 166 C 1200 900 2 15 Comparison 167 C 1200 900 10 0.1 Comparison 168 C 1200 900 10 25 Comparison 169 X 1250 900 50 0.5 Comparison

Microstructure observation was conducted on each steel sheet obtained above in the following procedures, and dynamic properties were evaluated by a tensile test and a hole expanding test.

Microstructure of each steel sheet was identified by the following method.

Each steel sheet was cut into a test piece for microstructure observation of 10 mm×10 mm×1.2 mm. The test piece was embedded in a cold rolled resin, and a position of ¼ of a sheet thickness in a rolled direction was observed. In such a case, impression as a landmark was formed with a Vickers tester so as to conduct identification on the structure observation place. The impression was corroded with repeller, and the structure was observed at five positions using an optical microscope with a magnification of 1,000. When the structure photograph after repeller corrosion was image analyzed, ferrite was observed gray, and martensite and retained austenite were observed white. After microstructure observation with an optical microscope, buff polishing and electrolysis polishing were conducted to an extent such that the Vickers impression does not disappear, and structure observation was conducted on the place at a step distance of 100 nm using FE/SEM-EBSP. A boundary having an orientation difference of 15° or more of grains was considered as grain boundary, and thus the grain boundary was identified.

Optical microphotograph and FE/SEM-EBSP structure photograph were combined as the standard of Vickers impression, and the structure was evaluated.

MD Structure:

The MD structure contains fine martensite or retained austenite (secondary phase) in ferrite grains as a matrix, as described above. The observation results of optical microscope and FE/SEM-EBSP were combined, a structure in which many martensite or retained austenite are present in the inside of grains surrounded by large tilt grain boundary (i.e., in the grains) was identified as MD structure, and its space factor was obtained.

There is the possibility that pearlite, bainite, ferrite which is not accompanied with martensite or retained austenite in grains, and the like are contained as other structures. Identification method of those structures is as follow.

Pearlite:

Pearlite has a lamella structure of ferrite and cementite, and the cementite can be identified with FE/SEM-EBSP. In the Examples, the space factor of pearlite in the observation view was determined.

Bainite:

Bainite is that lath is present in ferrite grains identified with FE/SEM-EBSP. Structure in which lath is present in ferrite grains was judged as bainite, and space factor of bainite in the observation view was determined.

Ferrite which is not Accompanied with Martensite or Retained Austenite in Grains:

Structure in which martensite or retained austenite cannot be confirmed in ferrite grains identified with FE/SEM-EBSP is considered as ferrite which is not accompanied with martensite or retained austenite in grains, and is judged as a structure different from MD structure. Space factor of ferrite which is not accompanied with martensite or retained austenite in grains in the observation view was determined.

Structure in which MD structure is not observed in the observation view is judged that MD is not present. When the whole structure is defined as 100%, in the case that the total of space factors of other structures (pearlite, bainite, ferrite which is not accompanied with martensite or retained austenite in grains, and the like) exceeds 10%, the defined structure is not formed. In this case, it was judged that MD is partial.

Proportion that secondary phase in the entire phase occupies in the whole structure:

Average Grain Size of Ferrite in the Entire Phase:

Proportion of Secondary Phase Present in Ferrite Grains in MD Structure:

First, the space factor of the secondary phase (martensite and retained austenite) in the whole structure used a space factor of white portion in the whole structure by image analyzing an optical microphotograph. Next, an average grain size (diameter of a corresponding circle) of ferrite was obtained using an image of FE/SEM-EBSP, and by combining with the optical microphotograph, the secondary phase present in the MD structure was separated into the secondary phase present in ferrite grains and the secondary phase present at grain boundary, thereby the space factor of the secondary phase present in grains of the whole secondary phase present in the MD structure was obtained.

Proportion that Retained Austenite in the Whole Structure Occupies in the Whole Structure:

On the other hand, martensite and retained austenite can be distinguished by identifying retained austenite having an FCC structure with FE/SEM-EBSP. However, retained austenite is an important structure in the present invention, and quantitative measurement of the space factor is indispensable to effectively exhibit the effect of the present invention. For this reason, a method of measuring a space factor of retained austenite by analyzing an image of FE/SEM-EBSP is considered, but in the Examples, the space factor of retained austenite was calculated by a saturated magnetization method which can obtain a measurement value of further high accuracy. It is considered that the value measured by the saturated magnetization method is a volume fraction, but the volume fraction can be regarded as being equivalent with an area fraction.

Average Grain Size and Number of Secondary Phase in the Whole Structure:

On the other hand, the average grain size (average particle size of secondary phase particles) and the number of the secondary phase (martensite and retained austenite) were obtained by the following methods. Regarding the average grain size, five places of observation view of 20 μm×20 μm are randomly extracted from SEM observation photograph (magnification; 3,000), average grain sizes (diameters of corresponding circles) of martensite and retained austenite in the respective observation views are obtained, and the average value thereof was used as an average grain size of martensite and retained austenite in the whole structure. Regarding the number, five places of observation view of 50 μm×50 μm are randomly extracted from the above optical microphotograph, and the numbers of the secondary phase particles observed thereon were averaged.

Various dynamic properties of steel were obtained by the following methods.

Tensile Properties:

Using JIS No. 5 test piece obtained from a vertical direction of a steel sheet rolled direction, tensile strength (TS), uniform elongation (u-EL), local elongation (1-EL) and total elongation (t-EL=u-EL+1-EL) were measured according to JIS Z 2241. Test pieces having TS of 590 MPa or more and t-EL of 10% or more were considered as acceptable products. Strain amount of from the maximum load point to breakage was considered as local elongation.

Stretch Flangeability:

Hole expanding ratio λ was measured as stretch flangeability. The hole expanding ratio λ was measured according to JISF (The Japan Iron and Steel Federation) standard (JFST 1001-1996). Steels having λ of 80% or more were considered as acceptable products.

The results obtained are shown in Tables 18 and 19 below.

TABLE 18 Measurement results Structure parameter Af of second Average structure Average grain Af of in grain size of Number second α grain size of secondary of Vf of γ Presence Af of structure in α in phase secondary in or MD in entire MD entire in entire phase entire Test Steel absence structure structure structure structure structure in entire structure No. code of MD (%) (%) (%) (μm) (μm) structure (%) 141 A 96.6 28.5 78.4 11.3 0.4 153 0.5 142 B 95.8 26.1 78.7 11.7 0.6 144 0.3 143 C 96.9 25.8 74.5 9.6 0.3 168 0.8 144 D 95.3 29.4 74.0 9.2 1.4 104 8.8 145 E None 146 F None 147 G 97.1 27.3 71.9 6.8 0.7 139 7.3 148 H 96.5 25.4 74.9 9.5 0.5 161 1.0 149 I None 150 J Partial 68.7 17.3 73.4 13.5 0.7 121 1.7 151 K Partial 83.1 31.7 72.5 12.3 3.2 134 0.7 152 L Partial 86.5 22.7 70.2 11.6 0.6 140 0.0 153 M 95.6 24.9 75.5 9.9 0.4 166 0.8 154 N 94.2 26.4 72.9 7.8 0.4 159 1.0 155 O Partial 65.8 62.8 76.2 11.2 4.1 73 0.8 Dynamic properties Total Uniform Local Test TS elongation elongation elongation λ No. (MPa) (%) (%) (%) (%) Remarks 141 870 25.8 9.2 16.6 101.7 Invention 142 618 29.5 12.1 17.4 103.4 Invention 143 784 27.7 13.9 13.8 93.9 Invention 144 1205 22.8 9.9 12.9 95.1 Invention 145 1298 4.9 1.8 3.1 19.3 Comparison 146 992 9.6 7.4 2.2 45.2 Comparison 147 892 27.4 15.8 11.6 88.6 Invention 148 811 26.9 12.7 14.2 96.2 Invention 149 1318 3.1 1.0 2.1 17.5 Comparison 150 433 30.4 21.9 8.5 67.5 Comparison 151 1179 8.4 3.4 5.0 31.3 Comparison 152 682 9.5 7.7 1.8 32.7 Comparison 153 844 26.2 13.2 13.0 94.2 Invention 154 852 24.9 12.8 12.1 93.4 Invention 155 1158 8.5 4.2 4.3 30.6 Comparison α: Ferrite, γ: Retained austenite, Af: Area fraction, Vf: Volume fraction, TS: Tensile strength, λ: Hole expanding ratio

TABLE 19 Measurement results Structure parameter Af of second Average structure Average grain Af of in grain size of Number second α grain size of secondary of Vf of γ Presence Af of structure in α in phase secondary in or MD in entire MD entire in entire phase entire Test Steel absence structure structure structure structure structure in entire structure No. code of MD (%) (%) (%) (μm) (μm) structure (%) 156 P Partial 63.2 16.4 72.5 13.8 0.7 131 1.7 157 Q 95.5 25.9 76.3 10.1 0.7 149 0.9 158 R Partial 88.7 24.1 75.2 10.9 0.7 141 0.9 159 S 95.6 28.3 70.4 5.4 0.3 172 4.2 160 T 95.2 27.8 71.0 5.9 0.4 169 4.1 161 U 93.8 25.9 73.9 9.5 0.5 158 5.8 162 V 96.7 25.7 75.4 8.6 0.6 146 5.1 163 W Partial 87.4 22.8 71.6 11.9 0.6 138 1.5 164 C Partial 56.7 11.6 72.4 17.5 0.8 99 0.8 165 C 94.9 27.5 78.3 22.4 1.1 118 0.8 166 C Partial 71.1 57.3 72.7 15.8 3.6 103 1.1 167 C Partial 60.4 16.5 70.1 17.2 0.7 74 0.9 168 C None 169 X 92.4 9.2 45.8 8.5 0.6 82 0.0 Dynamic properties Total Uniform Local Test TS elongation elongation elongation λ No. (MPa) (%) (%) (%) (%) Remarks 156 519 27.7 18.5 9.2 64.7 Comparison 157 993 23.4 12.8 10.6 92.8 Invention 158 850 18.6 15.0 3.6 36.2 Comparison 159 987 28.6 18.4 10.2 99.3 Invention 160 963 28.5 18.3 10.2 100.1 Invention 161 940 25.4 15.8 9.6 89.9 Invention 162 699 29.3 18.5 10.8 97.5 Invention 163 512 27.0 16.7 10.3 89.5 Comparison 164 719 24.8 15.9 8.9 78.9 Comparison 165 767 23.9 14.8 9.1 76.3 Comparison 166 1087 10.4 8.4 2.0 33.8 Comparison 167 630 26.9 18.8 8.1 77.3 Comparison 168 1302 3.2 0.9 2.3 21.9 Comparison 169 487 32.2 18.6 13.6 93.2 Comparison α: Ferrite, γ: Retained austenite, Af: Area fraction, Vf: Volume fraction, TS: Tensile strength, λ: Hole expanding ratio

Test Nos. 141, 142, 143, 144, 147, 148, 153, 154, 157, 159, 160, 161 and 162 each were satisfied with the provision of the present invention in structure morphology, and showed good properties in all of tensile strength TS, total elongation t-EL and hole expanding ratio λ. Average grain sizes of the secondary phases are all in a range of 3 μm or less.

On the other hand, No. 145 used steel E having C content larger than the range defined in the present application. As a result, nearly the entire surface was converted into martensite structure, and total elongation and λ were deficient.

No. 146 used steel F having Si content smaller than the range defined in the present application. As a result, the structure of bainite+martensite was formed, and total elongation and λ were deficient.

No. 149 used steel I having Si content larger than the range defined in the present application. As a result, nearly the entire surface was converted into martensite structure, and total elongation and λ were deficient.

No. 150 used steel J having Mn content smaller than the range defined in the present application. As a result, hardenability was poor, and ferrite was precipitated. As a result, TS and λ were deficient.

No. 151 used steel K having Mn content larger than the range defined in the present application. As a result, the structure of MD+martensite was formed, and total elongation and λ were deficient.

No. 152 used steel L having Al content smaller than the range defined in the present application. As a result, compounds of Si, Mn and O were unavoidably formed, and total elongation and λ were deficient.

No. 155 used steel O having Mo content larger than the range defined in the present application. As a result, the structure of MD+martensite was formed, and total elongation and λ were deficient.

No. 156 used steel P having B content smaller than the range defined in the present application. As a result, ferrite was precipitated, and TS and λ were deficient.

No. 158 used steel R having B content larger than the range defined in the present application. As a result, deterioration of λ which was considered due to grain segregation was generated.

No. 163 used steel W having C content smaller than the c range defined in the present application. As a result, ferrite was precipitated, and TS was deficient.

No. 164 was that the processing temperature T2 is lower than the range defined in the present application. As a result, ferrite was precipitated, and λ was decreased.

No. 165 was that the processing temperature T2 is higher than the range defined in the present application. As a result, the average grain size of the matrix ferrite is larger than the prescribed range, and λ was decreased.

No. 166 was that rolling reduction is lower than the range defined in the present application. As a result, bainite transformation is not accelerated, thereby forming the structure of MD+martensite. As a result, λ was decreased.

No. 167 was that the cooling rate was lower than the range defined in the present application. As a result, ferrite was precipitated, and λ was decreased.

No. 168 was that the cooling rate is higher than the range defined in the present application. As a result, martensite structure was formed, and total elongation and λ were decreased.

No. 169 used steel X having C content smaller than the range defined in the present application. As a result, the proportion of the secondary phase in the whole structure was reduced, and TS was decreased.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof.

This application is based on Japanese Patent Application No. 2006-283517 filed Oct. 18, 2006 and Japanese Patent Application No. 2006-283579 filed Oct. 18, 2006, the entire contents thereof being hereby incorporated by reference.

Further, all references cited herein are incorporated in their entireties. 

1. A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3 μm or less.
 2. The high strength steel sheet as claimed in claim 1, wherein the secondary phase is constituted of martensite.
 3. The high strength steel sheet as claimed in claim 1, wherein the secondary phase is constituted of martensite and retained austenite.
 4. The high strength steel sheet as claimed in claim 2, wherein an average grain size of ferrite in the whole structure is 20 μm or less, and wherein 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.
 5. The high strength steel sheet as claimed in claim 3, wherein an average grain size of ferrite in the whole structure is 20 μm or less, and wherein 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.
 6. The high strength steel sheet as claimed in claim 5, wherein the proportion that the retained austenite present in the whole structure occupies in the whole structure is 2% or more.
 7. The high strength steel sheet as claimed in claim 2, comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.
 8. The high strength steel sheet as claimed in claim 3, comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.
 9. The high strength steel sheet as claimed in claim 7, further comprising Mo: 0.03 to 1%.
 10. The high strength steel sheet as claimed in claim 8, further comprising Mo: 0.03 to 1%.
 11. The high strength steel sheet as claimed in claim 7, further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%.
 12. The high strength steel sheet as claimed in claim 8, further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%.
 13. The high strength steel sheet as claimed in claim 7, further comprising at least one of Ni: 0.5% or less, and Cu: 0.5% or less.
 14. The high strength steel sheet as claimed in claim 8, further comprising at least one of Ni: 0.5% or less, and Cu: 0.5% or less.
 15. The high strength steel sheet as claimed in claim 7, further comprising Cr: 1.5% or less.
 16. The high strength steel sheet as claimed in claim 8, further comprising Cr: 1.5% or less.
 17. The high strength steel sheet as claimed in claim 7, further comprising at least one of Ca: 0.003% or less, and REM: 0.003% or less.
 18. The high strength steel sheet as claimed in claim 8, further comprising at least one of Ca: 0.003% or less, and REM: 0.003% or less.
 19. A method for producing a high strength steel sheet, which comprises: heating a steel sheet material comprising, in terms of % by mass, C: 0.02 to 0.3%, Si: 0.01 to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities; and cooling the steel sheet material from a temperature of A3 point or higher to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec.
 20. A method for producing a high strength steel sheet, which comprises: cooling, from a temperature of A3 point or higher, a steel sheet material comprising, in terms of % by mass, C: 0.02 to 0.3%, Si: 0.01 to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities; subjecting said material to a processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C.; and then cooling said material to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec. 